The explosion happened at about 8:20 a.m. at the facility in the 6300 block of Cuba Valley Road in the town of Vienna, Dane County Sheriff's spokeswoman Elise Schaffer said.

Schaffer said the digester had been emptied Monday. When workers started an exhaust fan Wednesday morning, they reported hearing a loud noise. The nylon inflatable cover at the top of the digester then burst into flames.

Waunakee Fire Chief Gary Acker confirmed that the gas explosion happened, and said it was caused by a malfunction inside the plant.

"There is no threat to the local community," said Leo Maney, project manager with Clear Horizons. "The digester will remain down until we are confident it can be restarted."